Prior art structures have been developed specifically for use in actuators. These actuators employ a structure similar to that shown in FIG. 2. For example, it is known that an electrostatic linear motor can be fabricated. One major problem with these devices is that, as the actuation voltage on the structure is increased in order to move it in the X or Y planes, the undesired force in the Z direction pulls the movable structure down into contact with the electrodes below, thereby preventing any further motion. Existing research has focussed on methods of making the Z spring much stiffer than the X or Y spring. However, this typically requires that the springs have a very high aspect ratio, which is difficult to manufacture, resulting in a much more expensive MEMS device.
Because of the problems inherent with these types of devices, this invention is directed to a method for electrically flattening thin film movable mechanical structures and significantly improving their Z-axis mechanical stiffness, as well as compensating for any rotations about the X or Y axes. This method results in a significant reduction in manufacturing cost by making thin-film MEMS devices suitable for inertial sensing applications that would otherwise have required either thick-film MEMS devices or bulk silicon MEMS devices.
For purposes of this disclosure, the invention will be described in terms of a moveable MEMS structure that can be used as an accelerometer, however, the invention is equally applicable to any device having single or multiple moveable MEMS structures, located either beside each other or stacked along the Z axis.
It is well known to those of ordinary skill in the art that one way to detect motion of a moveable MEMS structure, is to apply a small amplitude high frequency periodic signal to it. The amplitude of this signal coupled onto adjacent stationary or moveable electrodes varies with the position of the MEMS device. The high frequency signal can either be imposed on the moveable MEMS structure, or onto nearby electrodes. In the case of the accelerometer example described herein, there are two sets of stationary interdigitated electrodes, which are finger-shaped, as well as a set of finger-shaped electrodes attached to the moveable MEMS structure. The capacitance between the finger-shaped electrodes on the movable MEMS structure and the nearby stationary finger-shaped electrodes can be measured by observing either the current, voltage or charge induced on those conductors by the high frequency signal applied to the movable MEMS structure. This is typically done by using a charge sensing amplifier, which is well known in the art. In this way, both the lateral position of the movable MEMS structure and its height along the Z axis above the finger-shaped electrodes can be measured, as the capacitance varies with the motion of the moveable MEMS structure. Measuring such capacitance variations in order to estimate the separation between conductors is well understood in the state of the art in MEMS.
To exemplify the invention, top and bottom electrodes have been added to the basic prior art structure illustrated in FIG. 4, thereby providing the capability of generating both upward and downward forces on the movable MEMS structure using purely attractive electrostatic forces.
The essence of the invention is as follows. A voltage is placed on the top electrode. This will generate an upward force on all parts of the movable structure. We can then apply a common voltage to the finger-shaped electrodes. This will generate a significant downward force on the movable MEMS structure. By sensing the Z height at one or many points on the movable MEMS structure and adjusting the common voltage on the finger-shaped electrodes, we can use feedback to keep the Z separation between the movable MEMS structure and the finger-shaped electrodes at a constant value. This will reject or servo out mechanical vibrations, accelerations, and rotations, even when the thin film itself is not stiff in the Z dimension. This method can be used to cancel out linear acceleration in the direction of the Z axis. Additionally, by varying the potential applied to the upper finger-shaped electrodes relative to the lower ones, the structure can also reject rotation about the X axis.
To reject rotations about the Y axis, electrodes under the movable MEMS structure have been added. Each of these electrodes is used to sense the separation between itself and the movable MEMS structure and to adjust the respective voltages in order to hold that separation constant. In this way, rotations about the Y axis will be removed by these feedback loops.
In addition to rejecting accelerations in the Z axis and rotations about the X and Y axes, the structure shown in FIG. 3 will also tend to flatten out the movable MEMS structure because it is sensing and servoing to a fixed value the Z axis height of the movable MEMS structure in several separate regions.
By applying a voltage difference between the fingers, a force can be generated in the X direction to cancel X acceleration. Note that the structure shown as an example of this invention cannot sense or cancel Y acceleration. To accomplish this would require the addition of horizontal fingers.
Currently in the prior art, cross axis sensitivity would have dramatically limited the applications and sensitivity that could have been achieved. However, by making use of this invention, extremely sensitive motion sensing MEMS devices can be manufactured using low cost thin-film fabrication techniques, and, in addition, these devices can have low Z axis sensitivity.